Group III nitrides such as GaN (gallium nitride) based semiconductors are expected as materials for next generation power semiconductor devices. A GaN-based semiconductor has a larger band gap than Si (silicon). For this reason, compared to Si (silicon) semiconductor devices, GaN-based semiconductor devices can realize small and high-voltage power semiconductor devices. In addition, since a parasitic capacitance can be reduced by this, a high-speed drive power semiconductor device can be realized.
Generally, in a GaN-based transistor, a high electron mobility transistor (HEMT) structure using two-dimensional electron gas (2DEG) as a carrier is applied. A normal HEMT is a normally-on transistor which conducts without applying a voltage to a gate. Therefore, there is a problem that it is difficult to realize a normally-off transistor which does not conduct unless a voltage is applied to a gate.
In such as power supply circuits dealing with large electric power of several hundred voltage to one thousand voltage, normally-off operation is required with emphasis on safety. Therefore, a circuit configuration is proposed in which a normally-on GaN-based transistor and a normally-off Si transistor are cascode-connected to realize a normally-off operation.
A measure is required against erroneous turn-on that, even though the transistor is turned off, due to the influence of other transistors switching on and off, the transistor turns on when charge flows into a gate of the transistor via feedback capacitance of the transistor. As one of the measures, a mirror clamp circuit is proposed which fixes the gate potential of a transistor.